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Abstract

Understanding of the tectonic evolution of the California Continental Borderland has developed relatively slowly because of the region's complex geology and its submarine character. For more than a century, scientists have recognized the dynamic character of borderland geology and tectonics—noting its basin and range physiography, identifying faults along steep linear escarpments bounding the offshore islands and submarine banks, and measuring earthquakes within the region. Exploration for offshore petroleum and other mineral resources with attendant technological developments in earth sciences generated maps of the subbottom geologic structure and confirmed interpretations of late Cenozoic deformation. The plate-tectonics paradigm provided an overall framework that constrained models of the regional tectonic evolution, while explaining the diverse geologic observations within this complex submarine province; borderland norma] faulting, of basin and range character, was replaced by dextral strike-slip faulting associated with the modern San Andreas fault system.

Yet, more than half of the entire region remains unexplored in detail, particularly that part west of Baja California, Mexico. Increased understanding of borderland tectonics requires a multidisciplinary and international effort. Recent technological advancements, including seafloor swath mapping and three-dimensional multichannel seismic-reflection profiling, allow major advancements in this tectonic understanding. The Miocene development of the Pacific-North America plate boundary involved the passage of the East Pacific Rise spreading center along the western edge of the borderland and the consequent lengthening of the dextral San Andreas transform-fault system. Broad-scale features of basin development and widespread volcanism during this epoch may be explained by regional oblique extension. Partly due to complexity associated with clockwise rotation of elongate crustal blocks and undefined deep crustal structure, palinspastic reconstructions are poorly constrained, and mechanisms of the plate-boundary reorganization remain to be described in detail. Recent detailed investigations of parts of the inner borderland reveal a systematic change from oblique extension in the southern region, west of northern Baja California, to oblique covergence in the north, west of southern California and the Transverse Ranges. This character is evident in both the late Cenozoic geologic structure, as mapped by seismic-reflection profiles, and in the present seismicity, as shown by earthquake focal mechanisms. Because its tectonic evolution includes each of the three major types of plate-boundary interaction—subduction, rifting, and transform faulting—the California Continental Borderland provides an exceptional opportunity for studies of these kinds of processes along an active continental margin.

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